There's no way to track direct replies to specific posts, you can only track overall new posts in a thread. I don't actually know of any forum software that allows tracking and notification of direct replies outside of the social networking world.

As for looking for threads with new replies: Thread lists are always sorted by last reply, so the thread with the most recent activity will always be on top (unless you specifically click a column header to sort by something else).

thanks dude. Do you think it is possible they could implement such a feature in the forum? In my opinion it would be quite useful